1. Field of the Invention
This invention relates to a combination process for demetalation and desulfurization of residua and gasification of coke thereby resulting. More particularly, this invention relates to a combination process for residua demetalation and desulfurization which comprises contacting said residua with a porous refractory oxide in the absence of added hydrogen and gasification of coke formed thereby on the refractory oxide by contacting said refractory oxide with steam and a free oxygen containing gas.
2. Description of Prior Art
Residual petroleum oil fractions containing relatively high proportions of metals, such as those heavy fractions produced by atmospheric and vacuum crude distillation columns, would represent excellent charge stocks for a cracking process were it not for their high metals content. Principal metal contaminants are nickel and vanadium, with iron and copper also sometimes present. Additionally, trace amounts of zinc and sodium may be present. Since these metals, when present in crude oil, are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all the metals present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organometallic complexes such as metal prophyrins.
At present, catalytic cracking operations are performed on petroleum fractions lighter than residual fractions. Typical cracking charge stocks are coker and/or crude unit gas oil, vacuum tower overhead, etc., the feedstock having an API gravity range of between about 15 and about 45. Such charge stocks do not contain significant proportions of the heavy and large molecules in which the metals are concentrated.
When metals are present in a cracking unit charge stock, such metals are deposited on the cracking catalyst. The metals act as a catalyst poison and greatly decrease the efficiency of the cracking process by altering the catalyst so that it promotes increased hydrogen production and coke formation.
The amount of metals present in a given hydrocarbon stream is generally judged by petroleum engineers by making reference to a charge stock's "metal factor", where said factor (F) is equal to the summation of the metal concentration in parts per million of iron and vanadium plus ten times the amount of nickel and copper. The factor may be expressed in an equation form as follows: EQU F = Fe + V + 10 (Ni + Cu)
A charge stock having a metal factor greater than about 3 is indicative of a charge stock which will poison cracking catalyst to a significant degree. A typical Kuwait crude, generally considered of average metals content, has a metal factor of about 150 to about 200. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that metals removal of over 90% will be required to make such fractions suitable for cracking charge stocks.
Sulfur is also undesirable in a cracking process unit charge stock. The sulfur contributes to corrosion of the unit's mechanical equipment and creates difficulties in treating products and flue gases. At typical cracking conversion rates, about one-half of the sulfur charge to such a unit is converted to H.sub.2 S gas which must be removed from the gasoline product, usually by scrubbing with an amine stream. A large portion of the remaining sulfur is deposited on the cracking catalyst itself. When the catalyst is regenerated, at least a portion of this sulfur is oxidized to form SO.sub.2 gas which must be removed from the flue gas which is normally discharged into the atmosphere.
In the past, high molecular weight, e.g. residual, stocks containing sulfur and metals have often been processed in a coker to effectively remove metals and also some of the sulfur. However, there are limits to the amount of metals and sulfur which can be tolerated in the product coke if it is to be marketable. Hence, there is a considerable need to develop economically practicable means for effecting the removal and recovery of metallic and sulfur contaminants from high boiling fractions of petroleum oils so that conversion of such contaminated oils to more desirable product may be effectively accomplished. The present application is particularly concerned with the removal of metal and sulfur contaminants from residua and gasification of the coke thereby formed.
It has been proposed to improve the salability of high sulfur and metal content residual-containing petroleum oils by a variety of hydroprocessing methods, e.g. hydrodesulfurization and hydrodemetalation. However, difficulty has been experienced in achieving a commercially feasible catalytic hydroprocessing. Short catalyst life in such processes is manifested by inability of a catalyst to maintain a relatively high capability for desulfurizing charge stock with increasing quantities of coke and/or metallic contaminants deposited thereon which act as catalyst poisons. Satisfactory catalyst life can be obtained relatively easily with distillate oils, but is especially difficult to obtain in desulfurizing residual oils, since the asphaltenic or porphyrinic components of an oil, which tend to form disproportionate amounts of coke, are concentrated in the residual fractions of a petroleum oil, and since a relatively high proportion of the metallic contaminants that normally tend to poison catalysts are commonly found in the asphaltene components of the oil. Further, on a commercial scale, these processes are rather costly due to high hydrogen consumption levels and to the expensive compression and recycling equipment required for high pressure operations. Consequently, there is an economic incentive to develop an upgrading process that does not require added hydrogen.
U.S. Pat. No. 3,716,479 proposes demetalation of a hydrocarbon charge stock by contacting the charge stock with added hydrogen in the presence of a catalyst material derived from a manganese nodule.
By the combination process of the instant invention, economically practicable and effective demetalation/desulfurization of a residual oil is obtained in the absence of added hydrogen.